Lecture 14
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Homework 5, due next Thursday (October 20)
at the beginning of class for Midterm exam
review
Wireshark Project 3 posted, due next Thursday
(October 20)
Programming Project 3 posted, due following
Tuesday (October 25)
Questions?
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Outline
Chapter 4 - Advanced Internetworking
4.1 The Global Internet
4.2 Multicast
4.3 Multiprotocol label Switching (MPLS)
4.4 Routing among Mobile Devices
4.5 Summary
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Multicast
IP supports multicast to support one-to-many
(radio, news, updates) and many-to-many
(teleconf, gaming) communication.
IP multicast (MC) uses MC groups, where each
group has its own IP MC address.
A sending host sends a single copy of a packet to
the MC address. The network routers copy the
packet whenever it needs to be forwarded over
more than one link.
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Multicast
IP multicast was originally a many-to-many (any
source MC or ASM) model. A one-to-many MC
model (source-specific MC or SSM) was
developed in which a receiver specifies a MC
group and a specific host.
A host joins and leaves an MC group by using
(IPv4) Internet Group Management Protocol
(IGMP) or (IPv6) MC Listener Discovery (MLD) to
communicate to the local router.
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Multicast Addresses
IPv4 uses addresses 224.0.0.0 through
239.255.255.255 (Class D) for multicast. This
address range uses a 4-bit prefix (1110) leaving
28 bits to specify an MC group.
Ethernet supports multicast (in addition to unicast
and broadcast) but uses only 23 bits for an MC
address. 32 (25) IP MC addresses map into each
Ethernet MC address. After receiving an MC
packet a host must examine the entire IP address
to either accept or reject the MC packet.
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Multicast Routing
While a unicast propagates along a path, a
multicast propagates along a tree.
Multicast routing is the process by which the
multicast distribution tree is determined.
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Multicast Routing - DVMRP
The Distance Vector Multicast Routing Protocol
(DVMRP) is an extension of distance vector
routing to support multicast.
DVMRP is a flood-and-prune protocol. In a flood
protocol each router would copy and forward a
multicast packet along all links except the one on
which the packet arrived if and only if the packet
arrived over the link that is on the shortest path to
the source.
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Multicast Routing - DVMRP
Members of MC group G
circled in red
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Multicast Routing - DVMRP
Duplicate packets can still be sent to LANs
connected to more than one router. DVMRP
solves this by requiring one router on a LAN to be
designated as the parent. Only the parent
forwards packets to the LAN.
We want to prune networks that contain no
members of the MC group. This can be
accomplished by having each member of the
group periodically announce that it is a group
member. Parent routers can prune networks that
contain no members.
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Multicast Routing - PIM SM
DVMRP does not scale well and Protocol
Independent MC (PIM) was developed in
response.
There are dense and sparse modes (PIM-DM and
PIM-SM). PIM-SM is the dominant MC routing
protocol and is the only one discussed here.
In PIM-SM, routers join the MC tree by sending a
Join message to a special router known as the
rendezvous point (RP).
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Multicast Routing - PIM SM
Here R4 sends a join message to the RP. R2 will
forward MC traffic only along the path from RP to
R4. R5's Join will not propagate farther than R2.
R2 will add the path to R5 to the MC tree.
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Multicast Routing - PIM SM
For MC group G, each router looks at the Join
and creates a forwarding table entry for the
shared tree, called a (*, G) entry.
To send a message to the group a host sends a
packet addressed to the MC group to a local
designated router (DR) which encapsulates the
message in a Register message which is
tunneled to the RP. The RP removes the packet
and sends it out to the shared tree.
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Multicast Routing - PIM SM
R1 is the DR for
the host. The
message is
unicast from R1 to
RP at which point
it is multicast
along the
distribution tree.
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Multicast Routing - PIM SM
A host sending a lot of data to the MC group can
trigger construction of a source-specific tree
rooted at the DR.
A high data rate from a single source can trigger
construction of a more optimal source specific
tree that replaces the shared tree. The DR in this
case effectively replaces the RP. This tree can be
significantly shorter than the original shared tree.
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Multicast Routing - PIM SM
A source specific tree that uses the shared tree is
shown in (c). A shorter source specific tree that
replaces the shared tree is shown in (d).
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Multicast Routing - MSDP
PIM-SM is typically used only within a domain.
The Multicast Source Discovery Protocol (MSDP)
was developed to extend MC across domains that
use PIM-SIM.
Each domain has an RP with peer RPs in other
domains. An RP sends Source Active messages
periodically to its peers on behalf of the sources.
A peer can join the MC group by sending a Join
message to DR for the source.
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Multicast Routing - MSDP
Construction
of a source
specific MC
tree across
domains
using MSDP.
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Multicast Routing - PIM SSM
PIM was originally designed as a many-to-many
protocol. PIM source-specific multicast (PIMSSM) was developed to support the demand for a
one-to-many protocol.
PIM-SSM required changes only to IGMP rather
than PIM. In PIM-SSM a source specific tree is
constructed first, bypassing the construction of a
shared tree.
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Multicast Routing - BIDIR-PIM
Bidirectional PIM
(BIDIR-PIM) is an
enhancement to PIM
that is better suited for
many-to-many
multicasting within a
domain when the
senders and receivers
may be the same
(multiparty videoconference).
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Multiprotocol Label Switching (MPLS)
MPLS tries to combine some of the properties of
virtual circuits with those of a datagram network.
MPLS-enabled routers forward packets by
examining short, fixed-length labels.
MPLS is primarily used for (1) destination-based
forwarding, (2) explicit routing, and (3) virtual
private networks and tunnels.
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MPLS - Destination-Based Forwarding
Router R2 has
assigned and
advertised labels for
certain network
prefixes. Arriving
packets have labels
attached by R1 (the
label edge router or
LER).
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MPLS - Destination-Based Forwarding
The IP longest match lookup algorithm has been
replaced by exact match lookup.
MPLS allows internal ATM switches to be used as
IP routers. (This can be a cost effective way to
carry IP traffic on an existing ATM network.) The
switches are now called Label Switched Routers
(LSRs).
Each MPLS label is associated with a forwarding
equivalence class (FEC). In this example, the
FECs are network prefixes.
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MPLS - Destination-Based Forwarding
Conversion of an
overlay network that
uses ATM switches
to a peer network (no
hardware changes
are involved).
R1 now has one next
hop instead of five,
resulting in simpler
forwarding tables.
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MPLS – Explicit Routing
MPLS enabled routers
also allow a network
to use explicit routing
(similar to source
routing). R1 can use the Resource Reservation
Protocol (RSVP) to specify the R4-R5 path to R7.
R2 can specify the R6 path. R1 and R2 attach
different labels to their packets.
The FEC is now based on the source router
instead of the destination
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MPLS - Virtual Private Networks
and Tunnels
MPLS enabled routers allow tunnels through a
network that can carry layer 2 data (ATM cells,
Ethernet or Frame Relay frames).
IP tunnels can be used similarly, but MPLS
tunnels use a much shorter packet header.
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MPLS - Virtual Private Networks
and Tunnels
Tunneling of ATM cells through an MPLS tunnel.
A demux label (DL) identifies the virtual circuit.
The tunnel label (TL) is a standard MPLS label.
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MPLS - Virtual Private Networks
and Tunnels
An ISP can
use a
network of
MPLS
enabled
routers to
create private
networks for customers. A common infrastructure
is used, but each customer appears to have their
own private network.
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Routing among Mobile Devices
DHCP has been a key enabling technology that
has made wireless hotspots feasible. It provides
an IP address and the identities of a default router
and DNS server for new devices on a network.
This is adequate for a large class of applications.
Other applications would break if we assigned a
new IP address to a host when it moves to a new
network. For example, Voice over IP telephone
calls, when a device moves between hotspots or
switches from wireless to 3G
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Routing to Mobile Hosts (Mobile IP)
An example is shown
at the right. The
correspondent's
packets need to be rerouted to the receiver's
new network.
The Mobile IP group developed a procedure that
allows a host to keep its original IP address as it
moves between networks. Applications then
continue to work seamlessly.
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Routing to Mobile Hosts (Mobile IP)
Mobile IP requires special routers known as the
home agent and foreign agent respectively. The
home agent intercepts packets intended for the
mobile host and tunnels the packets to the foreign
agent. The foreign agent delivers the packets to
the host.
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Routing to Mobile Hosts (Mobile IP) Optimization
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Optimizing routes between correspondent node
and mobile nodes is referred to as the triangle
routing problem, since Mobile IP path takes two
side of a triangle rather than the direct path.
The solution is to tell the correspondent node
the care-of address of the mobile node. Then
the correspondent node can create its own
tunnel directly to the mobile node. In the best
case, if they are on the same network, the
packets are addressed directly.
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Routing to Mobile Hosts (Mobile IP) Mobility in IPv6
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Mobility is designed into IPv6. Any IPv6capable host can acquire an address whenever
it is attached to a foreign network and can act
as a foreign agent.
Packets to the care-of address can contain an
extension header with the home address. This
allows the mobile node to present the illusion
that its IP address is fixed to the higher protocol
layers.
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In-class Exercises
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Start Homework 5
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